Composition and method for selectively etching a silicon nitride film

ABSTRACT

The invention relates to an aqueous phosphoric acid etch bath composition with a readily soluble silicon containing composition. The baths are used in the etching step of composite semiconductor device manufacturing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aqueous phosphoric acid etchingsolutions used in the manufacture of semiconductor devices. Morespecifically, the invention provides an aqueous phosphoric acid etchingsolution that exhibits increased etch selectivity of silicon nitridefilms over silicon dioxide films in silicon nitride-silicon oxidecomposite semiconductor devices. The selective etching of siliconnitride over silicon dioxide is accomplished by use of an etch bathsolution containing aqueous phosphoric acid and a silicon containingcomposition, such as hexafluorosilicic acid; that is readily soluble inthe etch bath solution. For purposes of the invention "readily soluble"means that the silicon containing composition blends uniformly with theaqueous etch bath at room temperature. The etch bath solution does notcontain hydrofluoric acid, nitric acid or mixtures thereof orcompositions containing elements from Group I or II.

2. Description of Related Art

The fabrication of a composite semiconductor device consisting of aseries of layers typically requires several basic processing steps.These basic processing steps include; cleaning, oxidation(atmosphericand high pressure), lithography(photolithography, electron-beam, andx-ray), doping by diffusion and implantation, deposition(electron-gun,sputter, chemical vapor deposition(CVD), low--pressure chemical vapordeposition(LPCVD), and plasma), etching(wet chemical and dry or plasma)and testing. These general techniques are well known to those skilled inthe art.

The present invention provides an improved wet chemical etch method thatincreases the etch rate of silicon nitride and enhances the etchselectivity of a heated, aqueous phosphoric acid solution for siliconnitride relative to silicon dioxide in silicon nitride-silicon oxidecomposite semiconductor devices. This is accomplished by adding areadily soluble silicon containing composition, such as hexafuorosilicicacid to an aqueous, phosphoric acid containing etching bath solution.Unlike other wet etch methods using heated a phosphoric acid etch baththe silicon containing compositions of the present invention are readilysoluble in the phosphoric acid containing etching bath solution.Furthermore, the solutions of the present invention do not containhydrofluoric acid, nitric acid or mixtures thereof or compositionscontaining elements from Group I or II such as sodium metasilicate. Thepresence of sodium, magnesium and other Group I or II elements isundesirable in many semiconductor processing applications.

Phosphoric acid etching baths are known in the art. U.S. Pat. No.4,092,211 to Northern Telecom Ltd. discloses a process for etchingsilicon nitride through apertures in a silicon dioxide mask in a boilingphosphoric acid bath. The process reduces the rate at which silicondioxide is etched relative to the silicon nitride by adding a solublesilicate such as sodium metasilicate to the phosphoric acid or by addinga finely divided silica to the etching solution. According to the patentthe addition of the soluble silicate or silica retards the etch rate ofthe silicon dioxide without affecting the etch rate of the siliconnitride. Japanese patent 60137024 to Matsushita Electric discloses theremoval of a film formed from the by-products of etching a siliconnitride (Si₃ N₄) film that has previously been subjected to a heattreatment in the presence of moisture. The by-product residues,described in the patent by the formula Si_(x) N_(y) O_(z) are removed byetching with a phosphoric acid solution containing fluorine ions. Theobject of the patent is to remove layers of silicon oxide, siliconnitride, and the by-product film of the silicon nitride heat treatmentin an efficient manner. Japanese patent application JP 6349808 A toHitachi Ltd., discloses a process of suppressing the etch rate ofsilicon dioxide film during silicon nitride film removal. This isaccomplished according to the invention by using a phosphoric acidetching solution containing silicon. Ideally, the etching solutioncontains at least 50 ppm silicon. The level of the silicon in theetching bath is maintained by adding fresh etching solution from amake-up vessel. U.S. Pat. No. 5,310,457 to AT&T Bell Laboratoriesdiscloses a method of achieving high etch selectivity of both siliconnitride and silicon with respect to silicon dioxide by using an etchbath of phosphoric acid, hydrofluoric acid and nitric acid. According tothe patent the addition of hydrofluoric acid and nitric acid inconcentrations of less than 500 ppm, ". . . increases the polysiliconand silicon nitride etch rates by as much as several orders of magnitudeand 100 percent respectively." The effectiveness of the etching bath ismaintained by subsequent additions of hydrofluoric acid and nitric acid.U.S. Pat. No. 5,472,562 to AT&T Corporation discloses the addition of asilicon to an etch bath, comprising; phosphoric acid, hydrofluoric acidand nitric acid. The addition of the silicon to the etch bath is said toenhance the initial etch selectivity of silicon nitride or silicon withrespect to silicon oxide. The silicon added to the etch bath is solublein the bath. An example of a composition that is soluble ishexafluorosilicic acid.

None of the references disclose or suggest a phosphoric acid containingaqueous etch bath further containing composition containing silicon,such as hexafluorosilicic acid that is readily soluble in the aqueous,phosphoric etch bath and the selective etching capabilities of thisbath. The use of silicates such as those disclosed in U.S. Pat. No.4,092,211 are not contemplated as part of the present invention.

BRIEF SUMMARY OF THE INVENTION

It has been found that the etch rate and selectivity of the siliconnitride etch relative to silicon oxide could be enhanced by adding areadily soluble silicon containing composition to an aqueous, phosphoricacid containing etch bath where no hydrofluoric acid, nitric acid ormixtures thereof or compositions containing Group I or II elements areadded to the etch bath. It is a purpose of the present invention toprovide a composition that overcomes the disadvantages found in the art.The compositions of the present invention exhibit an increased etch ratefor silicon nitride and selectivity of etch of silicon nitride relativeto silicon dioxide. The compositions of the invention are easy toprepare because the components are readily soluble in one another. Afurther purpose of the invention is to provide a method for effectivelyincreasing the etch rate of silicon nitride relative to silicon dioxidein a wet chemical etching process.

DETAILED DESCRIPTION OF THE INVENTION

Silicon nitride is widely used in the semiconductor industry as abarrier or top layer to prevent diffusion of contaminates into thesemiconductor device. It is also used as a selective oxidation barrierin the Localized Oxidation of Silicon (LOCOS) process to allow oxidationto occur in required areas to provide transistor isolation. Siliconnitride is essentially inert, dense and difficult to etch. Hydrofluoricacid and buffered oxide etchants can be used but etch rates aregenerally slow even at high temperatures and a photoresist is oftenadversely effected by extreme etching conditions. Another problem isthat when phosphoric acid is used in the etching solution the phosphoricacid etches both the silicon dioxide and the silicon nitride. Thisco-etching is undesirable when the selective etching of silicon nitrideis required. The previously mentioned U.S. Pat. No. 5,472,562 teachesthat the addition of a soluble silicon compound to an etching solutionof phosphoric acid, hydrofluoric acid and nitric acid will aid in theselectivity of silicon nitride etching with respect to silicon andsilicon dioxide. However, the presence of the additional hydrofluoricacid and nitric acid is undesirable and in many cases detrimental to thesemiconductor process. Other processes disclosed in the art providestripping compositions which have silicon added to modify the etchingcharacteristics of the composition. Typically, this may be accomplishedby adding a solid silicon containing material such as a silicon wafer toa heated phosphoric acid solution. This process is undesirable becauseof the length of time required to digest the silicon wafer and thepresence of undissolved particles in the etching solution.

The present invention provides a method of enhancing the etch rate ofsilicon nitride relative to silicon dioxide in a composite semiconductordevice by etching the composite semiconductor device in a heated,aqueous solution containing phosphoric acid and a silicon containingcomposition that is readily soluble in the aqueous etching solution. Theinvention further provides an aqueous etching composition containingphosphoric acid and a silicon containing composition that is readilysoluble in the aqueous etching composition.

According to the invention silicon nitride can be selectively etchedrelative to silicon dioxide by carrying out the etching operation in aheated etching bath containing an aqueous, phosphoric acid solution anda silicon containing composition that is readly soluble in the etchbath. Commercial grade phosphoric acid can be used. Typically, thecommercially available phosphoric acid is available as 80% to 85%aqueous solutions. In a preferred embodiment electronic grade phosphoricacid solutions having a particle count below 100 particles/ml and wherethe size of the particles is less than or equal to 0.5 microns andmetallic ions are present in the acid in the low parts per million toparts per billion level per liter are used. Examples of these types ofelectronic grade phosphoric acids include CleanRoom® electronic gradechemicals, CleanRoom LP™, CleanRoom MB™, and CleanRoom GIGABIT® (GB)available from Ashland Chemical. No other acids such as hydrofluoricacid, nitric acid or mixtures thereof are added to the solution of theetching bath.

According to the invention a silicon containing composition is added tothe etching solution. This silicon containing composition is readilysoluble in the aqueous etching composition at room temperature. In apreferred embodiment the readily soluble silicon containing compositionis hexafluorosilicic acid (H₂ SiF₆). In a preferred embodiment thereadily soluble silicon containing composition is present in the etchbath solution in amounts of from about 16 ppm to about 500 ppm. Theconcentration of the readily soluble silicon containing composition isgiven in weight(parts per million silicon calculated from the siliconpresent in the readily soluble composition based on the total bathsolution). In a more preferred embodiment the readily soluble siliconcontaining composition is present in the etch bath solution in amountsof from about 16 ppm to about 200 ppm. In a most preferred embodimentthe readily soluble silicon containing composition is present in amountsof from about 100 ppm to about 200 ppm. Silicon containing compositionsfurther containing Group I or II elements in amounts other than ascontaminants are not present in the etch bath solution of the invention.An example of such an undesirable composition would be sodiummetasilicate.

The bath temperatures at which the etching is performed may be anytemperature at which etching of silicon nitride-silicon oxide is carriedout. This type of etching process is typically carried out at about 180°C. An advantage of the present invention is that the etching can becarried out at much lower temperatures. When an aqueous phosphoric acidetch bath according to the invention is used, etching may be effectivelyperformed at temperatures as low as 150° C. According to the invention,the bath temperatures at which etching is performed is preferably fromabout 150° C. to about 180° C. The temperature of the etch bath solutionis more preferably from about 150° C. to about 160° C.

Having thus described the invention, the following examples are setforth to further illustrate the invention.

Measuring Techniques

etch rates were determined by measuring nitride and oxide thicknessusing ellipsometry(Plasmos Ellipsometer).

uniformity was calculated as

    1-(Max-Min/2×Average)×100

with Max=maximun thickness, Min=minimum thickness, Average=averagethickness measured on 49 different positions on a wafer.

Wafer Preparation

wafers for determination of etch rates;

Si₃ N₄ -wafers: 200 nm LPCVD nitride on oxide barrier

100 nm thermal oxide

Experimental Procedure

All experiments were performed in a class 1000 clean room environment ina quartz beaker, heated by a hot plate. In each set of experimentsdifferent temperatures were used (150° C., 160° C., 170° C., and 180°C.) using the same bath heated progressively. During heating of thephosphoric acid the lid was not put on the vessel. When the bath reachedthe desired temperature the first etch rate of Si₃ N₄ was determined bydipping nitride wafers in the bath for 10 minutes. The etch rate ofoxide was determined by a second dip of thirty minutes. The phosphoricacid used in run 1 was a GB grade 85% aqueous solution available fromRiedel-de-Haen. The phosphoric acid used in runs 2 through 8 was a GBgrade 85% aqueous solution from Ashland Chemical. The phosphoric acidused in runs 9 through 12 was a MB grade electronic grade 85% aqueoussolution from Ashland Chemical. The hexafluorosilicic acid was a 25%aqueous solution. Run 11 containing silicon added in the form of SiO₂was prepared by adding 3 grams of SiO₂ to 100 ml of H₃ PO₄ and addingthat combination to an 8 liter phosphoric acid etch bath and allowing itto stand overnight. The SiO₂ did not completely dissolve on heating to150° C. after standing over night. Table I shows the results of 12 runsmade at varying temperatures. The recorded results include the etch rateand uniformity of etch for both Si₃ N₄ and SiO₂ and the selectivity ofetch.

                                      TABLE I                                     __________________________________________________________________________    Etch rate on nitride and oxide substrate and nitride-to-oxide                 selectivity                                                                                 Nitride     Oxide                                                          T  Etch rate                                                                           Uniformity                                                                          Etch rate                                                                           Uniformity                                    RUN                                                                              Additive                                                                              (° C.)                                                                    (nm/min)                                                                            (%)   (mn/min)                                                                            (%)   Selectivity                             __________________________________________________________________________     1         150                                                                               3.3 ± 0.14                                                                      97.5  0.13 ± 0                                                                         95.8  25.38                                              160                                                                              5.65 ± 0.07                                                                      97    0.22 ± 0.02                                                                      95    25.68                                              180                                                                              10.3 ± 0.07                                                                      94.4  1.07 ± 0                                                                         92     9.63                                    2         150                                                                              3.26  97.9  0.02  98.6  24.2                                               160                                                                              5.74  97.9  0.23  98.2  25.2                                               170                                                                              5.12  97.2  0.49  97.8  10.5                                               180                                                                              13.7  90    0.87  97.5  15.7                                     3         150                                                                              3.5   96.5  0.15  97.5  23.3                                               160                                                                              6.6   96.7  0.18  97.3  36.7                                               170                                                                              6.4   96.4  0.23  97.2  27.8                                               180                                                                              9.9   94.6  0.6   96.6  16.5                                     4  16 ppm H.sub.2 SiF.sub.6                                                             150                                                                              5.65 ± 0.07                                                                      96.5  0.17 ± 0                                                                         95.5  33.24                                              160                                                                              7.55 ± 0.07                                                                      95.7  0.25 ± 0.02                                                                      95.3  30.2                                               180                                                                              10.6 ± 0.2                                                                       93.3  0.72 ± 0.02                                                                      93    14.65                                    5  50 ppm H.sub.2 SiF.sub.6                                                             150                                                                              7.34  96.1  0.09  98.3  81.5                                               160                                                                              9.09  93.9  0.25  97.4  36.4                                               170                                                                              10.2  95    0.47  96.7  21.7                                               180                                                                              10.7  94.1  0.88  97.8  12.1                                     6 100 ppm H.sub.2 SiF.sub.6                                                             150                                                                              10.5  91.9  0.02  97.2  462.3                                              160                                                                              12.5  89    0.17  97.8  73.56                                              170                                                                              13.4  87    0.72  96.7  18.63                                              180                                                                              15.25 86    1.33  95.4  11.46                                    7 100 ppm H.sub.2 SiF.sub.6                                                             150                                                                              8.6   91.2  -0.01 97.6  ∞                                            160                                                                              10.0  94.0  0.19  97.5  52.6                                               170                                                                              11.7  94.4  0.46  97.6  25.4                                               180                                                                              13.7  81.9  0.82  96.9  16.7                                     8 150 ppm H.sub.2 SiF.sub.6                                                             150                                                                              8.1   67.1  -0.08 96.7  ∞                                            160                                                                              13.2  89.6  0.07  96.5  188                                                170                                                                              14.9  90.6  0.69  96.1  25.1                                               180                                                                              18.2  32.8  1.24  93.3  15                                       9 200 ppm H.sub.2 SiF.sub.6                                                             150                                                                              15.0  36.4  -0.34 97.8  ∞                                            160                                                                              9.3   83.1  0.01  96.3  930                                                170                                                                              15.4  92.3  0.43  97.1  35.8                                               180                                                                              14.0  80.8  0.96  95.2  14.6                                    10 500 ppm H.sub.2 SiF.sub.6                                                             150                                                                              (1)   (1)   -0.3  97.7  ∞                                            160                                                                              14.4  92.5  0.26  97.9  55                                                 170                                                                              14.0  97    1.02  96.4  14                                                 180                                                                              14.5  95.3  2.28  87.1  6.4                                     11 100 ppm SiO.sub.2                                                                     160                                                                              5.2   98.0  0.00  98.7  ∞                                            170                                                                              7.24  96.6  0.06  98.5  121                                                180                                                                              14.8  87.9  0.29  98.7  51                                      12 eq. ppm HF as in                                                                      150                                                                              13    86    2.4   82     5.45                                      100 ppm H.sub.2 SiF.sub.6                                                             160                                                                              14    84    1.89  89     7.77                                              170                                                                              14    82    2.1   88     6.92                                              180                                                                              19    20    2.24  89     8.74                                   __________________________________________________________________________

The addition of H₂ SiF₆ results in an increase in the etch rate of Si₃N₄ overall. The nitride-to-oxide selectivity decreased as thetemperature increased for both the phosphoric acid with no additive andthe baths containing hexafluorosilicic acid. The addition of HF to thephosphoric acid in run 12 shows an increase of both oxide and nitrideetch rate and a decrease in nitride-to-oxide etch selectivity. Thesilicon added as SiO₂ in run 11 had no appreciable effect on the etchrate of the nitride film. The etch rate of run 11 is comparable to theetch rate with only phosphoric acid present in the bath.

We claim:
 1. An aqueous etching composition, consisting essentiallyof;A. phosphoric acid and B. hexafluorosilicic acid.
 2. An aqueousetching composition as claimed in claim 1, where the silicon containingcomposition is present in amounts of from about 16 ppm to about 500 ppm.3. An aqueous etching composition as claimed in claim 1, where thephosphoric acid is an 85% aqueous solution.
 4. An aqueous etchingcomposition as claimed in claim 1, where the phosphoric acid is anelectronic grade 85% aqueous solution.
 5. An aqueous etchingcomposition, consisting essentially of;A. an 85% aqueous solution ofelectronic grade phosphoric acid, and B. from 16 ppm to 500 ppmhexafluorosilicic acid.